December 11, 2012 | 5
Southern conifer species have had it easy over the past 65 million years — especially compared to their hard-knock northern kin.
Unlike their Southern Hemisphere cousins, whose northward-drifting continents continued to host stable habitats with cozy, Club-Med like conditions similar to those in which these conifers evolved, northern hemisphere conifer species were forced to cope with colder, drier, more seasonal conditions while doing the latitudinal lambada with pulsing glaciers.
As a result, new species replaced older ones more frequently in the north, and the average age of conifer species in the Northern hemisphere is several million years younger than in the southern hemisphere, according to a paper published in PNAS in October.
In this study, conducted by scientists from Yale, Utah State, and Harvard’s Arnold Arboretum, scientists compared the sequences of genes in these trees to construct a phylogeny, or family tree, showing the approximate time each species evolved. They compared this to the conifer fossil record, which helps us know the actual dates at which certain plant features evolved and which is relatively rich, since stiff plant parts preserve well.
They found that the northern hemisphere conifers had more recent divergence times than those in the Southern hemisphere. The median “node age”, or age of group divergence, was 5.2 million years for northern hemisphere groups and 8.7 million years for those in the south. Most Northern Hemisphere species evolved within the last five million years and many groups had median node ages around 3.5 million years ago. Conifer groups with younger species were not found to be correlated to latitude or particular habitats. Instead, the hemispheric association was strongest.
During most of the Mesozoic, or Age of Dinosaurs, in which most major modern lineages of conifers evolved, conditions were largely warm and wet. After the meteor strike that brought that era to a close, the climate of the Northern hemisphere became colder and drier. This alone may have favored the evolution of species more tolerant of new conditions.
At the same time, continents in the southern hemisphere were drifting north toward the equator. By a geographical happenstance, little of their landmass was in the glacial zone near the pole (with the obvious exception of Antarctica), and many pockets of warm, wet forest of the sort in which southern hemisphere conifers evolved persisted even to today.
Meanwhile, about 2.6 million years ago, Earth fell into a schizophrenic cycle of glaciations and brief intervening warm periods induced by the tilt and orbit of Earth (the famous Milankovitch Cycles). We are currently in one of those brief intervening warm periods scientists call “interglacials”. The last glaciation lasted about 110,000 years and ended about 10,000 years ago; civilization evolved 5,000 years ago. If we manage to survive our current game of chicken with CO2-induced warming, we are due for another 100,000-year Big Chill in perhaps 10-50,000 years. One problem at a time.
In addition to differences in their changing climates, major differences in the geography of the two hemispheres also produced differences in conifer evolution in those hemispheres. You can see these differences in these two graphics, which show the maximum extent of glaciers during the last glaciation in gray.
Here’s the Northern Hemisphere:
Here’s the Southern:
As you can see, conifers living in Canada had a long way to travel to get out of the way. The repeated north-south movement of such trees — and accompanying range expansion and contraction — in response to the creeping glaciers probably led to situations in which populations of trees were isolated in valleys or other areas separated from their kin.
Such isolation is a classic incubator for evolution. The most famous examples are seen on islands, where populations cut off from the mainland can evolve in radically new ways in relatively short times. In addition, many conifers in the northern hemisphere dwell in mountain ranges, where sharp altitude differences also select for more species living within close horizontal — although not necessarily elevational — proximity.
Meanwhile, southern hemisphere trees*, drifting north toward Margaritaville along with their continents, often lived in climates moderated by oceans and were largely not in the path of glaciers thanks to the chance arrangement of land. Although the Southern hemisphere also experienced climate change during the last 65 million years toward more open, dry environments (see the Continent of Australia), temperate rainforests and broadleaf evergreen forests that support older conifer lineages have persisted there much more commonly than in the Northern hemisphere.
That geography, and not just climate change, can have a major influence on the trajectory of evolution on different hemispheres of the same planet is a really interesting finding. Now that glaciers of all sorts seem to be the endangered species, it is likely that both Northern and Southern Hemisphere conifers will be under pressure to evolve once more, though whether geographic differences in the hemispheres will affect their future evolution is anyone’s guess.
* The lineages that gave rise to modern conifers were different in northern and southern hemispheres. In the north, the pines, firs, spruces, larches, hemlocks, firs, junipers, and cypresses evolved from two major groups: the Cypress Family (Cupressaceae — junipers and cypresses) and the Pine family (Pinaceae — everything else).
Southern conifers belong to the Araucariaceae and Podocarpaceae families and the subfamily Callitroideae within the Cypress Family. They are members of the Antarctic Flora, which I discussed here — plants that evolved on the former southern supercontinent Gondwana, which broke up to form the continents that host the southern hemisphere conifers.
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